Changes in the performance of semiconductor devices are a concern for systems with prolonged service lives that contain electronics that are not easily accessible for testing and replacement. Such effects are especially difficult to assess for devices that must be exposed to, and function in, transient ionizing radiation environments.
The use of infrared laser light to simulate the electrical effects of exposure to far more penetrating ionizing radiations, such as x-rays, gamma-rays, energetic electrons or single ions, is an established means of evaluating the localized radiation response of semiconductor devices. See J. S. Melinger et al., J. Appl. Phys. 84 (2), 690 (1998). Lower power, near-infrared bench-top exposure systems have been developed that track relative changes in semiconductor device dose-rate response based upon the fact that it is charge produced in the first few tens of microns of a silicon device that affects the electrically active regions of most modern semiconductor devices. See U.S. Pat. Nos. 7,019,311 and 7,375,332, which are incorporated herein by reference. In devices with moderately to heavily doped substrates, electron-hole pairs produced deeper in the device by more penetrating radiations, recombine before they can diffuse to near-surface circuit structures. Consequently, levels of charge generation comparable to that produced by far more penetrating radiation can be generated in the near-surface, electrically active regions of a device with low power near-infrared laser exposures.
In particular, laser dose-rate testing provides an efficient means to quantitatively track changes in the dose-rate response of semiconductor devices in systems with prolonged service lives. Accordingly, both focused and broad-beam, laser-based, dose-rate measurement techniques have been used to detect and track the relative changes in the dose-rate response of aging semiconductor devices. The purpose of the focused laser measurement system is to quantitatively and non-destructively document the position-dependent charge collection response of a semiconductor device (e.g., silicon device) to a specified radiation exposure. See U.S. Pat. No. 7,019,311. Using scanned, pulsed, focused, 904 nm laser exposures, the magnitude and time-delay of the peak charge collection across a device die is imaged with micrometer-resolution. Each laser pulse produces a concentration of electron-hole pairs within the electrically-active regions of the exposure area that is equivalent to a specified radiation environment. As the device die is rastered beneath the pulsed laser beam, the charge collection magnitude and time-delay measured at each exposure point is recorded to form quantitative, two-dimensional response images that can also be rendered as charge collection spectra (i.e., histograms) to create a response “signature” for the device. Device characterization with the focused laser system is complemented by a broad-area, pulsed, 904 nm laser exposure system that illuminates the full device die. See U.S. Pat. No. 7,375,332. The purpose of this system is to quantify and track the full die's functional response to ionizing radiation. The full die response can be recorded as a peak transient current, a change in the functional output of the device, or other performance parameter.
However, unlike x-rays, gamma-rays, or energetic electrons or ions, laser light is blocked from penetrating into near-surface regions where a device is overlain by opaque die metallization. This effect of shadowing by opaque metallizations is an inherent limitation in laser testing. Therefore, the usefulness of these laser techniques for making absolute measurements of a device's dose-rate response to a specified radiation environment has been limited by the fact that device die metallizations block the laser-illumination of the semiconductor material (e.g., silicon) that lies directly beneath the opaque metal, and thereby prevents the full replication of the charge generation effects that occur with more penetrating radiations. For example, the extent of this shadowing can be seen from the charge collection image of a 54LS14 integrated circuit shown in FIG. 1. FIG. 1(a) shows the position-dependent charge collection response of a portion of a 54LS14 die (corresponding to a single logic inverter among the six inverters fabricated in the die) as spatially imaged with a focused laser system. FIG. 1(b) shows the same region optically imaged with differential interference contrast microscopy. The outlines of distinct electrical diffusions are clearly visible in spite of the overlying metallizations. Diffused resistors 11 can be distinguished from arsenic electrical diffusions 14 and from unpatterned silicon 13 and other diffused regions 12 of the device.
In usual practice, dose-rate equivalent laser measurements are calibrated to electron or x-ray exposures by adjusting the intensity of the incident laser beam so as to produce the same measured peak transient current from the device as was measured during an electron or x-ray exposure of the same duration and known dose rate. Once the dose-rate response of a device has been qualified by traditional x-ray or linac-based tests, the laser-based measurements can be used to establish a baseline device response for relative comparison with subsequent laser measurements of an aging device. However, this calibration approach implicitly results in a higher density of charge generation in the exposed silicon regions of the device since no charge can be produced by the laser in the silicon covered by opaque metallizations. For example, if one-half of the surface area of a simple PIN diode were covered by opaque metallization, the incident laser would need to generate twice as much charge in the exposed silicon in order to attain the same overall peak transient current from the die as would be produced by an equivalent dose-rate exposure of electrons or x-rays that could penetrate through the metal. Thus, to the extent that an incident laser beam is blocked by opaque metallizations on the device die, the generation of charge is correspondingly reduced, and must be compensated for by either (a) increasing the incident laser intensity and thereby the corresponding charge generation within the exposed silicon, or (b) generating the same electron-hole pair density in the exposed silicon as would occur under electron or x-ray irradiation and then correcting the resulting measurement for the presence of occluded silicon areas.
However, the correction for the “missing” charge generation is not a simple multiplicative factor corresponding to the fractional area of metal coverage on the die; the correction factor must be weighted by the charge collection efficiency of the silicon lying beneath the metal. (In other words, metal that covers regions of low charge collection requires less “correction” than metal that covers regions of high charge collection.) As is visible in the micron-resolution, laser dose-rate response images shown in FIG. 1(a), charge collection can exhibit strong spatial dependence that must be factored into the metal-correction factor derivation.
Therefore, a need remains for a method to determine the position-dependant metal correction factor for dose-rate equivalent laser testing of semiconductor devices. Such a method would enable the determination of the absolute peak transient current response of a semiconductor device to ionizing radiation using bench-top laser diagnostic techniques.